The invention relates to a method and an apparatus for determining the transit time of echoes, triggered by exposing the bottom of the body of water to sound waves using sonic pulses, and more particularly to such a method and apparatus in which the echoes in received signals intercepted via at least one receiving lobe are detected.
In the case of depth sounding, such a method uses an echo or vertical sounder that has a receiving lobe pointing in the vertical direction or a receiving beam, or in the base of acoustical mapping of the surface profile of the bottom of the body of water the method uses what is known as a fan sounder, which spreads out a fan of receiving lobes or receiving beams in its sector, arranged in rows crosswise to the direction of travel of a watercraft and pivoted with respect to the vertical by a reception angle. In order to attain the smallest possible absolute measuring errors at great sounding depths, such a method must be in a position to determine the echo transit times with quite small error tolerances. A substantial contribution to this is made by the exact detection of the echo reflected from the bottom in the signals received directionally by the receiver.
In known methods of this kind, the echo detection is performed using the conventional methods of edge or threshold value discrimination. These methods are accurate only when the sending pulse meets the bottom exactly vertically. Then the echo is a reproduction of the sonic pulse itself, and the edge or threshold value descrimination leads to a very accurate temporal detection of the echo in the received signal. Contrarily, if a sonic pulse strikes the bottom at an angle of incidence that deviates from the vertical, then the received echo is no longer a reproduction of the sonic pulse that was sent, but is instead a roll-off function of the sending pulse over the bottom, received through a directional receiving lobe. As a result, discrete echoes, the echo length of which corresponds to the sending pulse length, are no longer received, but rather a distribution function via the receiving lobe is received. With such a distribution function, however, the methods of edge or threshold value discrimination are defective. Such roll-off functions, for one thing, always arise with a vertical receiving lobe if the bottom has an oblique course, that is, if it has an upward or downward slope with respect to a horizontal plane, and for another, arises in principle with receiving lobes pivoted counter to the vertical by an angle of reception, with the single exception being if the receiving direction is precisely normal to an inclined bottom.
Additionally, there is another phenomenon, which is dictated by the technical limits of side lobe suppression of the receiving antenna. Because sending pulses striking the bottom of the body of water or the sea bed normally are reflected back in the direction of sending or receiving, but sending pulses striking the bottom obliquely are dispersed or scattered, the amplitude of the vertical echo is considerably greater than that of the oblique echo. As a result, a fraction of the vertical echo is always contained in the received signal, and because of its great amplitude, even if it is suppressed, it is received via the side lobe. This vertical or side lobe echo can in many cases cause a response of the threshold value discriminator, and thus can result in extreme falsification of the echo detection. This happens for received signals both from directional receiving lobes that are inclined relative to the vertical and those from the vertical directional receiving lobe, if in the latter case the inclination of the bottom of the body of water is greater than the tangent of one-half the opening angle of the vertical directional receiving lobe. With a typical opening width of the vertical sounder of approximately 4.degree., this would be an inclination or drop of the bottom of the body of water of greater than 3.5%.